To obtain a desired realism, many flight simulators use a digital image generator (hereinafter DIG) to provide a trainee pilot with a view out of the window of a mock cockpit. For maximum training capability, such a DIG ideally provides imagery that is in correspondence with the view observed by the pilot during an actual flight.
It is impossible, however, in today's state of the art, to design and build a DIG that realizes this ideal capability. Nevertheless, it is possible to design and build a DIG that provides the trainee pilot with effective training cues.
To this end, it is desireable to select a particular DIG architecture that addresses the following design objectives. Thus, for example, the DIG should generate images that are free from anomalies or aliasing effects, so that the image does not exhibit stair-stepping, crawling, line breakup or scintillation.
To optimize the efficiency of the DIG, the selected DIG architecture should provide a mechanism that allows the DIG to operate near its maximum capacity. This means that even a properly designed machine will occasionally become overloaded. When this happens, however, the image quality should degrade gracefully.
Additionally, the DIG should be able to readily incorporate a texture capability, so that the trainee pilot is provided with important speed and altitude cases. Further, the DIG should be able to incorporate a translucency capability, so that clouds, smoke and dynamic shadowing effects can be introduced into a scene, thus enhancing its realism. A translucency capability, moreover, provides appropriate changes in the scene content of an image. Thus, when the amount of detail in the scene is changed, a new image is introduced gradually and imperceptibly, instead of "popping" into view.
As indicated above, it is desireable to select a particular DIG architecture that addresses the aforementioned design objectives. One important DIG architecture now being used that does not, in an entirely satisfactory manner, meet the required design objectives, is shown in FIG. 1. The DIG architecture of FIG. 1 is organized as a scanline based processor. This means that an image is generated scanline by scanline, synchronously with a displayed image. The basic operation of this processor is now set forth. A more thorough analysis may be found in the article "Computer Image Generation for Flight Simulation" by B. Schachter in Computer Graphics and Applications, October 1981.
Accordingly, the scanline based processor shown in FIG. 1 includes a geometric processor 10, a scanline computer 12, a video generator 14 and a display 16. The geometric processor 10 interfaces with a data base 18 which supplies the geometric processor 10 with the information that is used to process a particular image. In particular, this information includes a description of the objects that comprise the image. The objects, themselves, are described by a list of "faces" and the faces, in turn, are defined by a list of "edges".
The geometric processor 10, therefore, is supplied with a list of faces which describe objects. The geometric processor 10 acts on this list of faces and performs elimination of backward facing surfaces, geometric transformations and windowing. The geometric processor 10 also provides a tonal description (such as shading and fading) of each face. The geometric processor computations are stored in a memory (not shown) and finally transmitted to the scanline computer 12.
The scanline computer 12 uses the information transmitted from the geometric processor 10 to determine which faces are occulted and which faces are to be displayed on the display 16. In particular, the scanline computer 12 works on edge "intersections" and, as its name suggests, processes the edge intersection information serially, one display scanline at a time. Since the displayed image is generated scanline by scanline, this scanline based DIG system performance is in part limited by its ability to handle the most complex scanline. That is, the scanline based DIG is limited by a maximum number of edge intersections per scanline to produce an acceptable displayed image.
The output of the scanline computer 12, then, is a list of intersections for each scanline with the information for displaying the visible faces. This information includes the intensity, the color and (if appropriate) the parameters for smooth shading and atmospheric fading of the displayed image.
The output of the scanline computer 12, as may be observed in the processor shown in FIG. 1, provides an input for the video generator 14. In the video generator 14, the information for displaying the visible faces, which is supplied by the scanline computer 12, is transformed into picture-element-by-picture-element (i.e. pixel) information. In sum, the video generator 14 transfers the pixel information into a digital format that corresponds to the intensity of each displayed pixel. Finally, the video generator 14 provides a mechanism so that the digital pixel information may be converted into an analog electrical voltage or video signal which can be used to drive, in a raster format, the display 16.
The display 16 may include a conventional projector or TV-like cathode-ray tube (CRT). A typical CRT display encompasses a succession of equidistant scan lines, where each scan line is made up of pixels. The CRT constructs a displayed image by interlacing two separate "fields", where one field contains even-numbered scan lines and the other field contains odd-numbered scan lines. The interlaced fields are also called a "frame".
The basic operation of the scanline based DIG shown in FIG. 1 has now been set forth. Although this particular architecture provides an important advance in the art of computer image generation for flight simulation, it does not, as established above, fully satisfy the aforementioned design objectives. Notably, as indicated above, the scanline based DIG performance is limited in its ability to handle the most complex scanline. The limitation is due to the time available to process a scanline while maintaining synchronism with the display. This insufficiency leads to scanline overloads. In addition, this system has only a limited capability to suppress aliasing and popping.
The scanline based architecture of the DIG shown in FIG. 1 makes it difficult, moreover, to expand the basic system configuration from a single channel (or single view) system to a multi-channel system, without degrading the quality of the displayed image. Also, the architecture of this DIG makes it difficult to readily and inexpensively incorporate a texture and translucency capability.
The cited insufficiencies of the present DIG system architecture, as typified by the scanline based architecture shown in the DIG of FIG. 1, suggests that a need exists for a new DIG system architecture. The present invention provides such a new DIG system architecture that addresses the cited problems and improves upon the prior art digital image generators.
The new DIG system architecture of the present invention is a modular (or parallel) architecture of the type discussed, in general, in the article "A New Visual System Architecture" by R. A. Schumacker in Proceeding of 2nd Interservice/Industry Training Equipment Conference, November, 1980. The benefits of this new modular architecture include a closer adherence, than heretofore possible, to the design objectives described above. In particular, the benefits are realized because the modular architecture of the present invention is not limited in performance by a maximum number of scanline edge intersections. Additionally, the incorporation of translucency and texture capabilities as well as expansion to a multi-channel system, are readily and inexpensively realized by the modular architecture of the present invention. Finally, the apparatus of the present invention provides the desired realism that enables the trainee pilot to interpret varied, complex and frequently subtle visual cues.